This invention relates to a controlling means or assembly in a dual hydraulic brake system chiefly used in automotive vehicles, more particularly, to one wherein at least one circuit of the dual hydraulic brake system is provided with a hydraulic pressure controlling valve (hereinafter called a control valve).
In recent days, the automotive brake, hydraulically controlled one, is, for seeking a safer and surer braking effect in the midst of ever speed-increasing and worsening traffic conditions, taking a trend of separating the entire braking system into two mutually independent circuits, i.e., hydraulic circuits (hereinafter called circuit) connecting the master cylinder to each of the wheel cylinders disposed on the front and rear wheels. For example, a front brake circuit for the front wheel cylinders is independently separated, in one case, from a rear brake circuit for the rear wheel cylinders; in another case, a front wheel cylinder is involved with a diagonally apart rear wheel cylinder in an independent unit of circuit. All of these dual brake systems are mainly aimed at preventing failure of braking function, due to a trouble occurrence like an oil leakage, in all of wheels at a time, by completely separating the whole system into two circuits. In this type of dual braking system, a hydraulic control means (a regulating valve) is practically installed at least in one circuit of the two, for the purpose of attaining vehicle safety at the time of brake application, by means of dividing the braking power into two. In the previously mentioned system, separating the system into two circuits, i.e., one circuit to the front wheels and the other to the rear wheels, the hydraulic pressure generated in one of the two compressing chambers of the master cylinder, in response to the brake pedal depression, is delivered to the front wheel cylinders usually directly, and the pressure from the other compressing chamber of the master cylinder is indirectly delivered to the rear wheel cylinders, by means of inserting a regulating valve in the circuit, for the purpose of suitably adjusting the hydraulic pressure supplied to the rear wheel cylinders. On the rear wheel cylinders regulated hydraulic pressure is to be supplied for the sake of safety, not the direct and unregulated pressure. Specifically speaking, when the pressure delivered to the rear wheel cylinders should exceed by chance a predetermined value it is to be reduced smaller than the master-cylinder-producing pressure. This device, although ingenious to some extent, has been found still defective in that the braking power delivered in the rear wheel cylinders alone is, i.e. in case of a trouble occurrence in the circuit to the front wheels, insufficient for attaining a fully effective and safe braking of the vehicle.
For eliminating this disadvantage some other devices are proposed to be known, one of which is Jitsu-Ko-Sho-51 (1976)-29432 Japanese Utility Model, wherein when any trouble occurs in one circuit of the two a hydraulic control assembly, namely a regulating valve, disposed in the other circuit, is ineffectuated for fully actuating the very the other circuit to attain the desired increase of braking power. The technical concept of this proposition lies in providing a by-pass valve means, when a trouble happens in one circuit, for directly flowing the braking hydraulic fluid from the master cylinder, without passing through the regulating valve, to the wheel cylinders.
In this device the fluid can avoid the influence or adverse effects of the regulating valve, however, some innegligible side effects are unexpectedly observed, such as, extra increase of fluid amount for actuating the by-pass valve, complication of the structure incurring the increase of component parts for the by-pass valve, and further high precision required in the manufacturing of seals used therein.
Toyota Jidosha Kogyo Kabushiki Kaisha, the assignee of the present invention proposed before, on the other hand, still another device, which was patented by the U.S. Pat. No. 3,729,237 wherein an electrical detecting means sensing a remarkable pressure drop in one circuit actuates an electromagnetic restriction means to suspend the operation of the regulating valve. The patented invention further includes another proposition, in which the valve includes a first piston for regulating the braking fluid and a second piston, perpendicularly disposed to the first piston, for halting the movement of the first piston by a suitable shifting thereof. Although the proposition, which is provided with the second piston, has a great merit on one hand in minimizing the flow of the braking fluid caused by the shifting of the first piston, because this device is so constructed as to suspend the movement of the first piston by the second piston, it still can not be free from being faulty. Beacuse of its inherent construction mode, wherein the operation of the second piston executed on the balance relation of the hydraulic pressure between the mutually independent two circuits, it is likely to invite a shortage of the braking fluid supplied to the circuit in normal condition, when one circuit happens to be damaged. It results in insufficiency of the braking fluid delivered to the desired wheel cylinders, because the braking fluid in the normal circuit must be employed for actuating the second piston in addition to its proper purpose. This is not a minor problem but a serious one for the vehicle braking system, because the total fluid amount employed therein is extremely small in itself; even a slight amount of loss or shortage of the fluid in the normal circuit is innegligible. In addition, when one circuit is in malfunction the other normal circuit is strongly desired to be able to fully raise its pressure, so the fluid shortage in the very circuit urgently required of its capacity-up must be said critical. It often leads to a short of pedal stroke, or a so-called bottomming phenomenon, a forced pedaling shortage hindered by the abutment of the pedal onto the car floor.